This invention generally relates to reaction wheels used to provide attitude control for spacecraft, and more specifically applies to reaction wheel arrays.
Reaction wheels are commonly used to provide attitude control for a variety of spacecraft. Reaction wheels typically comprise a rotor, bearings and motor, with the reaction wheel coupled to the vehicle structure. The motor provides the ability to vary the wheel speed of the rotor. As the rotor speed is varied, a momentum exchange occurs and the motor provides a torque on the vehicle about the spin axis.
In most applications, multiple reaction wheels are used in a reaction wheel array. The multiple reaction wheels in the array are arranged so that their spin axes span three dimensions for three axis control. Arranging the multiple reaction wheels in this way allows the array to apply torque to the vehicle along different axes, generally all three. Torque can be selectively applied to these axes to provide attitude control of the vehicle.
There are several problems associated with reaction wheels that are commonly used today. A first problem is that reaction wheels generally have limited precision in their output torque. Typically, reaction wheels are designed to provide a specific maximum amount of torque. This maximum torque limits the resolution of smaller torques within the torque range by the minimum increment of command resolution. For instance, the digital electronics used to control the torque may impede precise control by limiting changes in command torque to a fixed number of steps (sometimes called the minimum torque impulse bit). For example, a reaction wheel that uses a 16-bit controller to control the commanded torque necessarily limits the output torque to one of 215 increments. Thus, it has been difficult with traditional reaction wheels to make more precise adjustments in the amount torque produced by the reaction wheel beyond these limitations.
Another problem in typical reaction wheels are the disturbances created by static friction as the reaction wheel speed goes through zero. As a reaction wheel approaches zero speed static friction becomes the characteristic form of friction, and causes disturbances that can be distributed throughout the vehicle as the wheel attempts to move away from zero speed. This commonly occurs when the reaction wheel changes direction of rotation. The region in which static friction creates disturbances is commonly referred to as the stiction region. These disturbances can interfere with the performance of the vehicle. For example, vibration in a satellite may prevent the satellite or its payload from accurately fixing on a desired target.
Another problem in typical reaction wheels are the disturbances created by the motors used to drive the reaction wheels and imperfections in the motor commutation circuits. These disturbances, typically referred to as torque ripple, are caused by the imperfect windings of the motor and commutation voltage offsets and gain mismatches in the electronics. As such, they generally have a frequency that is proportional to the rotational speed of the reaction wheel and amplitudes proportional to the output torque. These disturbances, like those caused by static friction, can interfere with the performance of the vehicle.
Each of these problems in current reaction wheel design can limit the functionality of the reaction wheel and the performance of the vehicle itself. Thus, what is needed is an improved reaction wheel system that minimizes these problems to provide an effective reaction wheel solution.
The present invention provides a reaction wheel system that includes at least two rotors. The first rotor is the primary rotor that provides the large output torques to the vehicle. The second rotor is a vernier control rotor that provides relatively small output torques and can be used to reduce the disturbances created by motor ripple, provide precise torque output control and/or reduce the disturbances created by static friction.
Specifically, the primary rotor and vernier control rotor each rotate about a common axis. The vernier control rotor comprises a rotor that is relatively smaller than the primary rotor, and rotates independently of the primary rotor. Because the vernier control rotor can be rotated independently from the primary rotor, it can be used to significantly improve the performance of the reaction wheel system.
For example, the vernier control rotor can be rotated to provide precise control of the output torque created by the reaction wheel system. In this example, the smaller vernier control rotor is used to augment the torque provided by the primary rotor, resulting in more precise control over the total torque created by the reaction wheel system.
In another example, the vernier control rotor can be rotated to improve momentum control when the primary rotor is operating in its stiction region. In this example, the vernier control rotor is rotated outside its stiction region, providing the required output torque until the primary rotor is outside its stiction region. Thus, the amount of disturbances created by static friction in the primary rotor and the reaction wheel system is reduced.
In a third example, the vernier control rotor can be rotated to minimize the disturbances created by motor ripple. In this example, the vernier control motor is configured to provide output torque that at least partially cancels the motor ripple created by the primary rotor motor. Thus, the amount of disturbances created by motor ripple is reduced.